The city's mean temperatures averaged 25.6 degrees in January - beating a high mark set in January 1991 by half a degree - and exceeding the long-run norm by 3.3 degrees.

It was also the highest temperature for any month in Sydney, and follows the hottest year set in 2016. The city's average maximum - at 29.6 degrees - shaded the record set in January 1896 by 0.1 degrees, and was also the hottest for any month, Agata Imielska, senior climatologist at the bureau, said.

The beach was the place to be for most of January in Sydney.

Photo: Kate Geraghty

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"We've seen back-to-back heatwave conditions," Ms Imielska said, adding that the lack of significant cool outbreaks, heavy rain and persistently warm offshore waters all played roles in the exceptional warmth.

With background conditions warming about a degree over the past century, climate change was also a factor.

"It's about looking at the absence of cool conditions, which ... is another trend with global warming," she said.

Similar factors were at play nationally, with last month becoming the third hottest on record for minimum temperatures. Mean and maximum readings were also above average but less notable, Ms Imielska said.

RFS crews battle a grass fire in Claremont Meadows on Tuesday as January came to a scorching end.

Photo: Wolter Peeters

Mild nights

While the city had its heat spikes - such as two days when 45.1 degrees were recorded at Badgerys Creek and Penrith Lakes, respectively - the month was also marked by its consistently mild nights.

During January, the mercury remained above 20 degrees for 25 nights, the most on record and almost triple the average of nine such nights. Meanwhile, 11 days topped 30 degrees at Observatory Hill, compared with the long-run norm of three such days.